1. Field of the Invention
The present invention relates to a fixing apparatus to be mounted in an image forming apparatus such as an electrophotographic printer or an electrophotographic copier.
2. Description of the Related Art
As a fixing apparatus to be mounted in an electrophotographic printer or copier, there is known a film heating type fixing apparatus. This type of the fixing apparatus includes a heater having a resistor heating element provided on a ceramic substrate, a fixing film that moves while coming into contact with the heater, and a pressure roller that forms a nip portion together with the heater through intermediation of the fixing film. A recording material bearing an unfixed toner image is sandwiched and conveyed in the nip portion while being heated, and hence the toner image on the recording material is fixed to the recording material. This fixing apparatus has an advantage in that a period necessary for the heater or the fixing film to reach temperature for fixing from start of power supply to the heater (warm-up period) is short.
Therefore, the printer including this fixing apparatus can reduce a period of time from an input of a print instruction to an output of a first copy of the image (first printout time or FPOT). In addition, this type of the fixing apparatus also has an advantage in that power consumption is small during waiting for a print instruction.
However, such a film heating type fixing apparatus is known to cause a phenomenon in which a friction force between the pressure roller and the fixing film is lowered by moisture condensation on an outer circumferential surface of the pressure roller so that the film cannot be rotated (hereinafter abbreviated as “condensation slip”).
In the film heating type fixing apparatus, the film and the heater having a small thermal capacity are heated fast, but the pressure roller is not easily heated because the pressure roller has a larger thermal capacity than the film and the heater. Therefore, when the recording material undergoes a fixing process at a timing when the film and the heater reach a target temperature for fixing, the pressure roller is not yet sufficiently heated so that water vapor generated from the recording material is apt to be condensed.
The above-mentioned condensation slip is described below. When the recording material bearing an unfixed toner image is guided into the nip portion, water vapor is generated from the recording material in the process of heating the unfixed toner image. When a surface of the pressure roller is not sufficiently heated, moisture is condensed on the surface of the pressure roller. The fixing film is rotated to follow rotation of the pressure roller by a frictional force between the outer circumferential surface of the fixing film and the surface of the pressure roller in the nip portion. Therefore, when the frictional force is reduced by an influence of the moisture adhering to the surface of the pressure roller, the fixing film may be decelerated or stopped. In the state where the fixing film is decelerated or stopped, it is difficult to convey the recording material. As a result, a sag of the recording material may cause abrasion of an image surface or a paper jam may occur in the fixing apparatus.
In order to suppress occurrence of the condensation slip, there is proposed a structure in Japanese Patent Application Laid-Open No. 2007-206275, in which a air supply fan (blower fan) is used to remove water vapor filled in a periphery of the pressure roller.
However, in the image forming apparatus described in Japanese Patent Application Laid-Open No. 2007-206275, so-called pressure roller contamination (pressure member contamination) may be generated, in which offset toner on the surface of the fixing film, paper powder of the recording material, or the like is adhered and accumulated on the surface of the pressure roller in the nip portion.
The adhesion of toner that can be a main cause of the pressure roller contamination becomes worse more easily as temperature of the surface of the pressure roller becomes lower. When the temperature of the surface of the pressure roller is high, the toner on the surface of the pressure roller is softened so as to adhere to the recording material being conveyed, and hence is removed from the surface of the pressure roller. However, when the temperature of the surface of the pressure roller is low, it is difficult to remove the toner on the surface of the pressure roller.
As described above, there is a problem in that, when the air supply fan is constantly driven as a countermeasure against the condensation slip, the temperature of the surface of the pressure roller is lowered so that the pressure roller contamination is generated easily. The pressure roller contamination causes irregularities of a fixed image, wrinkles of the recording material, tangling of the recording material to the pressure roller, and the like.